A surface light source device fitted with a so-called edge-light type lightguide arranged so as to illuminate by inputting light from a side end plane of a transparent plate and by emitting the light from the other plane thereof (light emitting surface) is used as a back illuminator of a liquid crystal display of a word-processor, a personal computer, a thin television set and the like. Such surface light source device is constructed such that a tubular light source is disposed at one side end plane, two side end planes perpendicular to each other or two side end planes opposing to each other of the lightguide in general and the remaining planes of the lightguide, except of the incident side end plane and the light emitting surface, and the back side of the tubular light source are covered by a light reflector.
In order to emit primary light inputted from the side end plane of the lightguide uniformly and efficiently from the whole light emitting surface of the lightguide, it is necessary to arrange the lightguide such that its scattering-reflecting capability is low around a light source and its light diffusing-reflecting capability is high at the region farthest from the light source and to distribute the light such that the most of the inputted primary light is emitted from the emitting surface in scattering and reflecting the light guided to the lightguide in the direction perpendicular to its traveling direction. Then, a variety of principles and processing methods for giving such scattering and reflecting capability have been proposed as described below and part of them have been put into practical use.
(1) Those Characterized Mainly by Rough Surface:
There are ones in which the whole light emitting surface or the surface facing thereto is roughened (See JP-A-3-118593, JP-A-118248, etc.), in which the roughness of the rough surface is changed (See JP-A-63-168604, etc.) or in which speckled or linear rough surface patterns are disposed and formed by changing its a real density (See JP-A-4-162002). The surface is roughened by sand-blasting or chemically etching a die and the surface rough patterns are formed by combining photo-etching and sand-blasting (See JP-A-4-52286).
(2) Those in Which Scattering Reflector is Applied:
Those in which a scattering-reflecting substance containing white paint or particles is applied in a meshed dot or linear pattern on the rear surface facing to the light emitting surface by means of screen printing or the like (See JP-A-57-12838, JP-A-1-245220, etc.). Its manufacturing process includes two steps of forming a specular plate having no pattern and of printing the patterns.
(3) Those Characterized Mainly by Diffusion of Bulk:
There have been disclosed ones in which lightguide bulk resin itself is adopted as a light diffusing-scattering substance by mixing light scattering particles, by blending non-compatible polymer or by co-polymerization (See JP-A-1-172801, JP-A-2-221924, JP-A-5-249319, JP-A-6-186560, etc.).
(4) Those Caused by Protruding or Concave Patterns:
These are divided roughly into the following three types. These are fabricated by machining a lightguide itself or a molding die by machine cutting, laser processing, die etching or the like.
1) Concave Patterns:
There are ones in which concave patterns are disposed on a light emitting surface or on a surface facing thereto on the datum plane. There are, for example, ones having one-dimensional linear triangle grooves (See JP-A-2-165504, JP-A-6-3526), rectangular grooves (See JP-A-6-123810, JP-A-6-265731, etc.), semi-circular grooves (See JP-A-5-79537U), broken-line shaped triangular grooves (See JP-A-5-196936, JP-A-5-216030, etc.) and others. There are also those having two-dimensional conical or pyramid engraving (See JP-A-4-278922), semispherical engraving (See JP-A-6-289393, etc.) and cylindrical engraving (See JP-A-1-145902U). There have been also proposed one in which the inner surface of a concave portion of the pattern is roughened (See JP-A-4-355408, JP-A-5-94802U, etc.).
2) Protruding Patterns:
There are ones in which protruding patterns are disposed on a light emitting surface or on a surface facing thereto on the datum plane. There are, for example, ones having one-dimensional linear triangle protrusions (See JP-A-5-313163, JP-A-6-75123), rectangular protrusions (See JP-A-5-79537U) and semi-circular protrusions (See JP-A-6-281928). There is also one having two-dimensional semispherical protrusions (See JP-A-5-79537U, JP-A-6-281929, etc.). There are also ones in which those protruding parts are roughened (See JP-A-5-94802U, JP-A-6-186562, etc.).
3) Concave and Protruding Patterns:
There are ones which have no flat plane on a light emitting surface or on a surface facing thereto and in which one-dimensional saw-tooth patterns or two-dimensional grid patterns are disposed. There are, for example, ones having the one-dimensional saw-tooth patterns (See JP-A-64-11203, JP-A-6-250024, etc.) and the two-dimensional grid patterns (See JP-A-62-278505, JP-A-3-189679, etc.) and ones whose surface is roughened as a whole (See JP-A-6-342159, JP-A-6-123885, etc.).
In addition to the demands on high luminance and high uniformity ratio of illuminance of the past, the demand on a large screen, thin-type, light-weight and low power consumption display is growing more and more lately. The trend is now shifting from a plate-like lightguide which has been put into practical use mainly by the printed patterns of (2) of the past to a tapered (wedge) type lightguide which is thinner and lighter. Then, the pattern printing step which has been required in the technology of (2) described above has become unnecessary and injection molding which allows scattering reflection patterns to be formed in the same time is considered to be desirable in terms of the cost.
From this aspect, the above-mentioned methods (1) through (4) have had the following various problems, respectively.
Although those having the uniform rough surface by the method (1) can be mass-produced by injection molding by using a die, it has had problems that the die is complicated because the die must be formed into a complicated curved wedge shape such that region where primary light enters is fully thickened and the region distant from the light source is thinned in order to attain uniform luminance as a surface light source, the degree of freedom of the shape of the lightguide is restricted and there is a limitation in increasing the area and thinning of the lightguide in principle. Further, although there has been a proposal of changing the surface roughness, it is very difficult to realize that. Meanwhile, although the method of distributing the rough surface as speckle or linear patterns is a relatively excellent method which allows the shape of the lightguide to be freely formed and the luminance to be uniformed in the pattern design, it is risky because of unstable elements in the die fabrication process such as variation between accuracy of photo-etching in forming the patterns and the surface roughening process such as blast in the next step in creating the die.
Although the printing method (2) has been the method put into the practical use most for the conventional plate type lightguides, it has had a problem, as a first problem, that it does not have a merit in terms of the cost as compared to the method of forming patterns in the same time by injection molding because the pattern printing step is a separate step. Further, it has had a problem that the pitch of dots of the scattering reflection patterns cannot be made smaller than around 1 mm (see JP-A-5-100118) due to the limit of the printing accuracy (see JP-A-4-289822). Further, due to this problem of the printing accuracy, the reproducibility of minimum points and minimum lines is low during the pattern printing, thus lowering the production yield and increasing the cost (see JP-A-3-9304, JP-A-4-278922). Still more, in terms of its performance, a difference of brightness occurs locally between regions where the pattern exists and where the pattern does not exist because the pattern is rough, thus causing luminous unevenness, so that it is a general practice to uniform the local unevenness of luminance caused by the rough pattern by providing a diffusing plate or a diffusing sheet having a high diffusing efficiency on the emitting surface side of the lightguide. However, because the diffusing plate or the diffusing sheet having a high diffusing efficiency has a low total ray transmission factor, it causes loss, thus lowering the luminance (see JP-A-5-100118, JP-A-6-265732).
In case when rough patterns are disposed orderly, moire occurs between a prism sheet or a liquid crystal panel disposed in order to increase luminance in the vertical direction on the emitting surface side of the lightguide due to the local luminous unevenness. Although methods of setting pattern intervals at random (see JP-A-5-313017 and JP-A-6-242442) or of disposing the patterns obliquely with respect to a ridge-line direction of the prism sheet (see JP-A-5-257144 and JP-A-6-230228) have been proposed in order to prevent that, they make it difficult to design or to assemble.
Although the method (3) allows mass-production by injection molding or the like and is anticipated to cause no local luminous unevenness at all due to the pattern in principle, it is considered to be difficult to achieve the uniform luminance just by the bulk scattering method. Further, it is not easy to give the distribution of the light diffusing performance to the lightguide bulk itself and is difficult to mass-produce. Further, it is necessary to change the thickness of the tapered shape or the like or to create concave and protruding patterns to realize the uniform luminance by a resin material of uniform diffusing agent. Thus, there is a possibility that it becomes complicated as it is required to use other means for achieving the uniform luminance or that the restriction on the shape of the lightguide becomes a big problem.
The method (4) is an excellent method in terms of mass-producibility when press or injection molding is used by using a die. As means for achieving the concave and protruding patterns, methods of using mechanical cutting, laser processing or chemical etching have been disclosed. However, as for the pattern shaping accuracy, the dimensional accuracy and the roughness of the plane where the patterns are created, all of the processing methods have had considerable difficulties and have been risky in terms of the processing stability, reproducibility and cost when the lightguide has minute patterns and has a large area. Lightguides which have been put into practical use so far have been small lightguides of several inch in size having pattern pitch of around 1 mm and the problem of the pattern roughness in (2) still exists. Further, ones in which a relatively large irregular plane is created and the irregular plane is roughened further has had problems that variation caused in the roughening process adds on the problem of the accuracy in creating the irregular plane, thus causing unstableness in fabricating a die and causing a problem in terms of the cost in fabricating the die.
Accordingly, it is an object of the present invention to provide a uniform, thin and large-area lightguide causing no local luminous unevenness or no luminous unevenness on the whole light emitting surface by emitting primary light incident on the lightguide efficiently from a light emitting surface.